1. Field of the Invention
The present invention relates to magnetic-head and magnetic-disk testers, particularly to improved testers of read-write heads and magnetic disks and to a method for compensating positioning errors which occur during the operation of such testers.
2. Description of Prior Art
Prior to assembling computer disk drives, their main components, i.e., magnetic read-write heads and magnetic disks, are usually checked on magnetic-head and disk testers.
An example of a prior-art head and disk tester is shown in FIG. 1, a schematic perspective view of the tester. Such tester is described in earlier U.S. Patent application Ser. No. 840,527 filed Feb. 25, 1992 now U.S. Pat. No. 5,254,946, granted Oct. 19, 1993. The tester comprises two parallel bottom rails 10 and 12 which are rigidly attached to a tester housing 13, e.g., by bolts (not shown) and support and guide top rails 14 and 16, respectively. Top rails 14 and 16 are rigidly attached to an outer ring 18 which rotatingly supports an inner ring 20. The latter carries a magnetic-head holder 21 with a magnetic head 22. Thus, magnetic head 22 can move back and forth in the directions indicated by an arrow A and can be rotated by means of inner ring 20 around an axis B, which passes through head 22. A spindle 24 rotates around a fixed axis and supports and rotates a magnetic disk 26 to be tested.
For positioning the head at a predetermined location of disk 26 and at a predetermined angle to the disk's track (not shown), top rails 14 and 16 are guided over bottom rails 10 and 12 and shift magnetic head 22 radially with respect to magnetic disk 26. Inner ring 20 is then rotated, thus placing magnetic head 22 in the required position and at a predetermined angle to the track.
The testing requires that magnetic head 22 and disk 26 be accurately positioned in a predetermined relationship with respect to each other. Among parameters to be measured, an important one is track profile. Track profile is a characteristic which is obtained by measuring a read-back signal parameter, such as amplitude, pulse shape, etc., which depends on the transverse shift of the magnetic head with respect to the center line of the track.
Outer ring 18, together with top rails 14 and 16, form a carriage 28 which is shown in FIG. 1. Carriage 28 is moved along bottom rails 10 and 12 from a reversible stepper motor via a lead screw (not shown).
The system shown in FIG. 1 is an open-loop system, i.e., it does not employ feedback. Typically, the width of the track is about 10 .mu.m or less. The open-loop system of this type produces a positioning uncertainty of about 1 .mu.m. However, with the advent of a new generation of hard disks with tracks narrower than 10 .mu.m, the uncertainty of 1 .mu.m becomes unacceptable for measurement purposes.
One method of improving positioning accuracy is to employ a closed-loop system, measuring the displacement of head 22 and actively correcting any positioning error. It is, however, very difficult to measure the displacement of head 22 directly by means of a measurement instrument. This is because the space adjacent to the head is occupied by spindle 24, magnetic disk 26, and means for removably mounting head 22 on carriage 28. Therefore there is not enough room for locating the measurement instrument such as a linear encoder.
An attempt has been made to solve this problem by providing a tester with a closed-loop system which incorporates a measuring instrument located in a different place than the center of the carriage, e.g., on the side of the carriage. An example of such a device is an experimental disk/head tester which was constructed by Guzik Technical Enterprises of San Jose, California in July 1992. A schematic three-dimensional view of this tester is shown in FIG. 2.
The tester of FIG. 2 is essentially the same as the one of FIG. 1, with the exception that a linear encoder 11 is installed in close proximity to one side of carriage 28a, e.g., near bottom rail 12a. Encoder 11 consists of two parts: a stationary part or a reader head 11a which is attached, e.g., to housing 13a by screws 15 and 17, and a movable part, i.e., encoder scale 11b which is rigidly attached to carriage 28a, e.g., to top rail 16a by screws 19 and 23.
When during testing carriage 28a moves together with magnetic head 22a relatively to magnetic disk 26a, encoder 11 is used for determining a position of magnetic head 22a. Encoder 11 does not measure the position of magnetic head 22a: it is assumed that the displacement of a measurement point on the side of the carriage 28a, where the encoder is located, is approximately equal to that of the center of magnetic head. However, this is not true, if yawing occurs during the movement of the carriage.
More specifically, in the case of yawing, magnetic head 22a is displaced with respect to the measurement point by being rotated around this point with a radius equivalent to the distance from magnetic head 22a to the measurement point. With the radius of 10 cm and a yawing angle, which may be as high as 5 .mu.radians, the displacement of magnetic head 22a with respect to the measurement point due to yawing, i.e., the position measurement error, may be as high as 0.5 .mu.m.
Therefore, the closed loop compensation system (not shown) of the tester of FIG. 2, which uses an output signal of encoder 11 as a feedback signal to compensate for the head-position error, cannot achieve an accuracy better than 0.5 .mu.m.